Mars is in excellent viewing right now so he does not need to wait. It'll only get better in October.

I do not understand the wisdom in leaving a cap over the telescope to view bright objects. The less light that hits the mirror, the less resolution you'll have, right? Doesn't seem like a good idea when using high magnification. A proper filter makes more sense to my brain.

I have used a 4mm eyepiece and have used a 2x barlowed 10mm eyepiece for my telescope and have not discerned any optical quality loss. None. I think that the concerns over barlow lenses may be an antiquated notion from an era of worse manufacturing quality or may be specific to more dim DSO's.